Verfahren und Vorrichtung zum Absichern eines Wechselstromkreises

A device (1a) pretests an inductance load for overload or short-circuit prior to connection to the power supply network. After power-on, voltages with small angles of advance or retard are applied to the load for pretesting the load, whereby the angles of advance or retard are gradually increased. At the start of every voltage application, the active current is measured in a narrow time window. If it does not exceed a preset active current limit value when a specific rms voltage is applied, the load is connected to the network. In other cases, the power-up process is interrupted. The power-up process avoids high loading of the network in the event of a short circuit or an overload. The active current measuring window starts when voltage is applied to the load. At an interval to the time window there is another measuring window for the reactive current, whereby the latter measuring window starts at the zero-crossing of the alternating current voltage. The measurement of the reactive cur rent helps to detect a defined magnetization end position of the inductance load in order to switch the load to the network at the correct phase and without inrush current (Fig. 2).